Nonporous membranes are barriers to flow, but are selectively permeable to certain species by diffusion. For example, a membrane separating two different fluids prevents gross or indiscriminate mixing of the fluids, but may permit preferential passage of one or more components of the fluids. Flux is a measure of the rate of passage, and selectivity is a measure of the discrimination shown by the membrane toward the various species that can pass through it. Certain polymers are used in membranes for gas separation, and in electrochemical applications such as fuel cells and electrolysis. In the latter case the polymers are ionomers, i.e., polymers with ion-exchange capacity.
Membranes may be in the form of polymer films, in which case they must have, in addition to properties suitable for acting as a barrier and for transport, sufficient strength to resist tearing or puncturing, or excessive stretching such as would be caused by differential pressure across the membrane. Alternatively, composite membranes are used in which the polymer is supported on a reinforcing substrate. In composites, no more polymer need be used than necessary for effective barrier and transport properties. Improved strength is provided by the reinforcement. Examples of reinforcing materials are woven or nonwoven fabric, or expanded polytetrafluoroethylene (ePTFE, available commercially, for example as Tetratex® or Goretex®).
In many application it is desirable that the composite membrane be thin, often on the order of micrometers. Such thin membranes can be made by coating ePTFE. A plurality of coats may be applied, often to both sides of the ePTFE. This can be done by spraying or dipping, but a continuous process would be preferred if uniform impregnation and uniform thickness could be assured without damage to the ePTFE, especially for thin, and therefore, fragile ePTFE and resulting composite membranes.
An improved process is needed for coating of ePTFE to make composite membranes.